Cementitious boards useful in the construction industry are known to contain inorganic, hydraulically setting material, such as Portland cement or gypsum. Hydraulic gypsum and cement, once set, have very little tensile strength and are usually reinforced with facing materials which improve the resistance to tensile and torsional loads. This has been the basis for using paper facing on conventional gypsum wall board and glass fiber scrim in cement boards.
Recently, gypsum sheathing has been used outdoors in exterior insulation or finishing systems, with or without insulation layers, (sometimes referred to as “EIF systems”). These systems are designed to accept polystyrene insulation adhered to a glass-faced gypsum board, followed by brick, stone, a thin application of stucco, or siding, for example. Because of the exposure to the elements, these boards are often treated or impregnated with hydrophobic additives. Simply adding hydrophobic agents, such as asphalt, waxes, or siloxane to the core or paper facing, however, does not prevent delamination at the interface between the core and paper facing, nor does it prevent the paper from losing most of its strength due to water absorption over time. Cementitious boards used in exterior sheathing applications should also be fire resistant, which limits the use of flammable facing materials. It has become essential, therefore, that the core, the interface, and the facing material, on the exposed side of these boards, retain their strength after exposure to wind, rain, snow, fire and ultraviolet light.
To the extent that cementitious boards are used in the interior of buildings, for tile backing, shaft wall, and fire wall applications, moisture and fire resistance requirements also limit the use of paper facing. While paper can be treated with moisture vapor retarders and non-permeable barrier films, these layers impede water vapor from cementitious slurries from evaporating through the facing. This makes continuous board manufacturing with vapor impermeable films difficult, since the slurry must be allowed to evaporate the excess “water of convenience”, which represents additional water used to facilitate a slurry, which is not used in hydration.
Accordingly, artisans over the last thirty years have developed facing materials made from glass fibers which are known to overcome many of the weaknesses of paper facing materials. For example, in Canadian Patent No. 993,779, gypsum boards are disclosed which are prepared by depositing a plaster slurry on a sheet of inorganic fibers on a moving conveyor belt. A second sheet of similar fibers is pressed on top of the slurry, followed by guiding the sandwiched plaster slurry between rollers to cause a slurry to penetrate into the fibrous sheets at the surfaces of the slurry mass. It has been found that such a process gives only partial and irregular penetration of the slurry into the inorganic, fibrous sheet, leaving the resulting board with a rough surface in which both fibers and gypsum are apparent. Due to the penetration of slurry, contamination of the rollers by set gypsum is an unfortunate consequence. Such contamination can leave imperfections in the surface of subsequently produced boards on the same line if not remedied. Unfortunately, remediating contaminated rollers requires shutting down the entire continuous wallboard manufacturing line for a period of hours or days, which leads to significant losses in production and downtime costs.
Others have addressed this problem of gypsum slurry penetration during continuous board production. In U.S. Pat. Nos. 3,993,822 and 4,504,533 (also incorporated herein by reference), a composite glass mat layer containing a fiberglass cloth joined with a fiberglass fleece, pasteboard, foil, felt or paper is used to make boards. In each method, the fleece, felt or alternative sheet is resistant to the penetration of gypsum and the structure is formed by simple successive application of the different sheets and compositions to a forming table or conveyor. The resulting product has a surface texture determined by the nature of the outer sheet used. However, due to the lack of any integral fastening measure between the glass fiber cloth and the outer sheet, the outer sheets selected can be insufficiently bonded to the set gypsum core. This can lead to unintended delaminations during adhesive bonding in the assembly of EIF systems. Polymeric webs combined with glass scrim reinforcement has also been suggested for facing sheets of cementitious boards. See U.S. Pat. No. 6,054,205, which is also hereby incorporated by reference.
More recently, attempts have been made to employ the use of random oriented, chopped glass fibrous mats bonded by a resinous adhesive. See U.S. Pat. Nos. 4,647,496, 4,810,569 and 5,220,762. The described production methods of these patents prefer to employ viscosity-control agents, such as paper fiber, cellulosic thickeners, bentonite clays, and starches for permitting the top glass fiber mat to be substantially free of set gypsum, while not preventing penetration through the lower mat. Since only one surface of the gypsum board needs to be free of set gypsum, for adhesive attachment of polystyrene insulation layers, and such, designing a slurry to penetrate the lower glass mat, while only partially penetrating the upper glass mat, produces a commercially acceptable product. However, the fouling of shaping and feeder rolls used to press and shape glass mat faced gypsum wallboard is not entirely solved by such a solution, since gypsum slurry penetration through the lower mat is not prevented.
While efforts have been made to reduce moisture penetration in glass mat faced gypsum boards by applying latex coatings to the set board (see U.S. Pat. No. 5,552,187, which is incorporated by reference) artisans have been reluctant to reduce the porosity of the glass mat prior to contacting the gypsum slurry. Two reasons for this could be the fear of insufficient penetration by the slurry into the pores of the mat, which would result in poor adhesion, or the concern that a less porous mat would prohibit or reduce water vapor from successfully escaping the core during setting.
Accordingly, there remains a need in the industry for manufacturing cementitious boards to create a high strength, fire-resistant facing material which does not delaminate in wet environments, yet is not substantially permeable to slurries of cementitious material which have been known to contaminate rolls used in the continuous processing of such boards.